Hydraulic control devices such as are known from German Patent Document DE 38 33 845 A1 are used to control two-cylinder viscous fluid pumps. The feed cylinders of these pumps are each driven by a hydraulic linear cylinder, work in a push-pull operation. In other words, one cylinder feeds, while the other--in the so-called suction stroke--is loaded with material to be transported, for example with concrete. The alternating opening of the feed chambers of the two feed cylinders toward the storage container containing the material to be transported and the alternating connecting of the feed chambers to a feed line is done by a tube switch. This switch controls both the function of an inlet valve for the feed cylinders which, through a changeover of the tube switch, moves into a communicating connection with the storage container, and also the function of an outlet valve for the feed cylinder carrying out the feeding stroke, the feed chamber of which feed cylinder being connected to the extended feed line. To changeover the tube switch to the two alternative feed paths, hydraulic linear cylinders acting in opposite directions are provided. The pistons of these cylinders are kinematically positively coupled with one another through the tube switch and can be reversed through an alternating pressure loading and pressure relief of the bottom-side control chambers of the two linear cylinders.
The feed line, which can be connected alternatingly to the feed chambers of the feed cylinder through the tube switch, can be blocked from the tube switch by means of a shutoff device controlling the function of the outlet valve. This shutoff device is a seated valve which can be moved into its blocking position by a bottom-side pressure loading of a hydraulic control cylinder, in which blocking position the feed line is blocked off from the tube switch. After the pressure relief of the bottom-side control chamber toward the tank of the pressure-supply plant, this valve is moved into its open position connecting the tube switch to the feed line by the pressure developing during the feeding operation in the respective feed cylinder in its feed chamber. To control the feeding operation of the viscous fluid pump, a sequence control is provided which reacts to hydraulic or electrical outlet signals from the end-position indicators which monitor the positions of the drive cylinders of the feed cylinders, the control cylinders for the tube switch and the control cylinder of the shutoff device. This sequence control ensures that the feed cylinders are actuated to carry out the feeding or loading stroke until after the tube switch has been moved into the operating position suited for the feeding and loading operation of the feed cylinders. The flow path, through which the pressure loading of the control cylinder in the sense of a closing movement of the shutoff device occurs, leads through a check valve, which is loaded in an opening direction by the high control pressure. The relief path, through which pressure medium can discharge from the bottom-side drive chamber of the control cylinder, is guided through a pressure-limiting valve which is connected in parallel to the check valve. This pressure-limiting valve is set in its open position as soon as the pressure--during the feeding operation--exceeds an adjustable presettable value. This is supposed to occur so that the shutoff device opens only when the pressure in the feed chamber of the respective feed cylinder operating in the feeding operation corresponds approximately to the pressure existing in the feed line in order to achieve a desired pre-compression of the material being transported in the feed chamber and to avoid uneven operation of the pump caused by the pressure differences between the feed chamber and the feed line.
This type of changeover of the shutoff device prevents the pressure in the bottom-side drive chamber from dropping below the pressure threshold specified by the setting of the pressure-limiting valve. This may not be advantageous as long as during the course of the pump operation the friction ratios do not change and the pre-compression pressure is sufficient to open the shutoff device against the pressure present at the shutoff device, which hereby, however, will not reach a definite opening position but a position of a dynamic pressure balance, that is, it will stop in a position between its closing position and the position of a maximum opening cross section. This position will be, in particular with the common arrangement of the feed line such that it branches laterally off from the valve chamber through which the piston rod of the control cylinder, which piston rod has the valve member, axially passes, a position in which the valve member is in the area of the port opening, which from a technical flow standpoint is much less favorable than when the piston can be moved on into an end position, in which the port opening of the feed line is completely open. A disadvantageous result of this is that a significant greater amount of driving energy is needed for the drive cylinder of the feed cylinder; in addition, the position, in which the control piston of the shutoff device reaches its balancing position, can also depend on the friction ratios in the drive system, with which a clear correction between the necessary pre-compression pressure and the pressure which is indeed needed to open the shutoff device, does not exist so that in particular when the friction ratios change during the course of a longer operation, an increasingly uneven run of the pump occurs. This can indeed be countered by reducing the reaction threshold value of the pressure-limiting valve which, however, is complicated.
If, on the other hand, an "active" opening of the shutoff device is achieved, for example, by loading the rod-side drive chamber of its control cylinder with pressure for opening the shutoff device, then a correlation between the pre-compression pressure and the opening pressure of the shutoff device does no longer exist, and it is at least difficult to achieve an adjustment of the pump and its control valves so that the shutoff device opens at least approximately at the pressure existing in the feed line, which is the important condition for being able to avoid compression strokes or decompression strokes during the changeover of the cylinders from the feeding to the loading operation. In addition, such an active opening of the shutoff device requires a significant amount of technical hydraulic switching and additional expense.